1. Field of the Invention
The present invention relates to a radio selective calling receiver and, more particularly, to a radio selective calling receiver having a battery saving function. The present invention also relates to a scanning type radio selective calling receiver.
2. Description of the Prior Art
In a radio selective calling receiver using a battery as a power supply, to prolong the service life of the battery, the power consumed by the reception section must be suppressed low. In addition, if a large-capacity battery is used, the overall radio selective calling receiver increases in size. For this reason, to reduce the size of the compact radio selective calling receiver, it is important to reduce the power consumption of the reception section.
Under the circumstances, in a general radio selective calling receiver, battery saving (to be referred to as BS hereinafter) control is performed such that the reception section is not always operated but is intermittently operated at predetermined time intervals to suppress the power consumption of the reception section low.
To suppress the power consumption of the reception section low, it is desired that the reception section be intermittently operated at long time intervals. However, excessively long time intervals increase the probability that the radio selective calling receiver fails to receive a radio signal which should be received. As a result, the reception function of the radio selective calling receiver may be impaired. For this reason, BS control in a general radio selective calling receiver is performed such that the reception section is intermittently operated at time intervals which are set to ensure the reception function.
Such BS control may be effective while the radio selective calling receiver is in the coverage zone in which radio signals can be received. If, however, the reception section is intermittently operated at such short time intervals even while the radio selective calling receiver stays in an area, outside the coverage zone, in which no radio signals arrive at the receiver, the power is wasted.
The radio selective calling receiver disclosed in Japanese Unexamined Patent Publication No. 4-304043 has been proposed as a means for solving such a problem. This radio selective calling receiver is designed to detect whether a sync signal is received. On the basis of this detection, it is checked whether the receiver is in the coverage zone. Upon detecting the movement of the radio selective calling receiver out of the coverage zone, the receiver switches the BS control modes to reduce the power consumption of the reception section. More specifically, when this radio selective calling receiver moves out of the coverage zone, the receiver starts counting a period of time that elapses upon moving out of the coverage zone. The reception section is intermittently operated at first time intervals which are relatively short time intervals until this elapsed time reaches a predetermined time A. After the elapsed time exceeds the predetermined time A, the reception section is intermittently operated at second time intervals which are longer than the first time intervals.
According to this radio selective calling receiver, since intermittent reception is performed at short time intervals when the time period of a stay outside the coverage zone is shorter than the predetermined time A, even a call signal that arrives immediately after the receiver returns to the coverage zone can be quickly received. If the time period during which the receiver stayed outside the coverage zone is longer than the predetermined time A, the time intervals of intermittent reception are switched to the long time intervals. With this operation, the power consumption of the reception section can be suppressed low, and hence the service life of the battery can be prolonged.
In the above conventional radio selective calling receiver, movement of the receiver out of the coverage zone is detected when no sync signal is received, and the time intervals of intermittent reception are uniformly prolonged. According to this BS control, a problem may be caused in performing synchronization establishment when the receiver returns to the coverage zone depending on the time intervals of intermittent reception after the movement of the receiver out of the coverage zone. This problem will be described below.
Consider two cases in which the radio selective calling receiver moves out of the coverage zone: the first case in which the receiver moves out of the transmission area in which radio waves transmitted from a transmitting station can be received; and the second case in which the receiver enters a place, e.g., a subway or a building, within the transmission area, in which radio waves are blocked.
In the first case, an abrupt operation for restoration to the coverage zone is rarely required. No problem is therefore posed even if the time intervals of intermittent reception outside the coverage zone are prolonged to a certain extent. In contrast to this, in the second case, quick restoration to the coverage zone and synchronization establishment for this purpose are required immediately after the blocked state of radio waves is canceled. If, therefore, the time intervals of intermittent reception outside the coverage zone are prolonged, such quick synchronization establishment is difficult to perform.
In the above conventional radio selective calling receiver, however, the time intervals of intermittent reception are uniformly switched without checking whether the movement of the receiver out of the coverage zone corresponds to the first or second case. For this reason, if priority is given to a reduction in power consumption by prolonging the timer intervals, a problem may be posed in quick synchronization establishment for restoration to the coverage zone. To prevent this problem, the time intervals of intermittent reception outside the coverage zone need to be shortened. If, however, the time intervals are shortened, the power consumption cannot be sufficiently reduced.
Demands have recently arisen for a receiver to which a plurality of frequencies at which a self-call number is transmitted are assigned in advance (to be referred to as a scanning receiver hereinafter). This type of receiver is used as a multi-area receiver which can automatically switch a plurality of areas with different frequencies. According to this receiver, since a plurality of frequencies at which the self-call number is transmitted are assigned to the receiver, the receiver searches for a sync signal in a transmission signal to be received while sequentially scanning the assigned frequencies when the receiver is outside the coverage zone.
Upon reception of the sync signal, the receiver stops scanning the frequencies, and locks the frequency to the above frequency. As a result, the receiver is set in the state in which a self-call signal can be received, i.e., the coverage-zone state. In the coverage-zone state, the frequency is kept fixed until a condition for determination of movement out of the coverage zone is satisfied. According to the scanning receiver, automatic multi-area selection is realized as follows. In the coverage-zone state, a frequency is fixed. When the receiver moves out of the coverage zone, synchronization with a transmission signal, which has been established in the coverage-zone state, is canceled, and a sync signal is detected while the frequencies are switched.
According to such automatic multi-area selection, for example, in the receiver disclosed in Japanese Unexamined Patent Publication No. 06-315001, in the out-of-coverage-zone state, frequency scanning is performed a predetermined number of times according to a predetermined pattern. If no sync signal is detected, frequency scanning is performed again according to the above pattern after a lapse of a predetermined period of time. A general condition of determination of movement out of the coverage zone is that no sync signal is consecutively received a predetermined number of times at the sync signal transmission timings in the coverage-zone state.
In such a conventional radio selective calling receiver as a scanning receiver, since a plurality of frequencies cannot be simultaneously received, the intervals at which a sync signal is detected at a specific frequency are prolonged in proportion to the number of scanning frequencies. That is, as the number of frequencies to be detected increases, detection of a sync signal delays even if the receiver is in the signal transmission area, and a longer period of time is required for a shift from the out-of-coverage-zone mode to the coverage-zone mode.
If the number of times of detection of a sync signal is simply increased, the BS effect deteriorates in the out-of-coverage-zone state although a detecting operation is performed at short intervals.
In the first case in which the above out-of-coverage-zone state is established, priority should be given to the BS effect. In the second case, synchronization must be quickly established. In contrast to this, in the conventional radio selective calling receiver disclosed in Japanese Unexamined Patent Publication No. 6-315001, predetermined BS control is performed after the receiver moves out of the coverage zone regardless of the immediately preceding reception state in the coverage zone. That is, the above two cases are not discriminated. For this reason, if priority is given to the BS effect, synchronization establishment delays. If priority is given to synchronization establishment, the BS effect in the out-of-coverage-zone state deteriorates.